In the bowl of a stand mixer, on medium high speed, (or use a hand mixer) mix together the egg yolks and granulated sugar.
Next add in cornstarch, salt and cocoa powder; lower speed on stand mixer.
Add the heavy whipping cream, Beiley's Irish Cream (or Creamer) and milk; mix together scraping down the sides and bottom to thoroughly blend mixture.
Transfer from the mixing bowl to a large pot, over medium heat, and whisk until mixture begins to boil; whisking the entire time is required (the mixture will burn very quickly if directions not followed).
Once filling has thickened significantly, turn off heat and swirl in butter and vanilla; whisk together until glossy and smooth.
Allow the cream to chill until it is barely warm.
After the filling has cooled a bit, add it to the cookie crust and smooth out until even.
Place the pizza in the refrigerator for a minimum of 3 hours or overnight.